A series of selection experiments with Tribolium castaneum as the experimental organism are being used to investigate (a) the importance of overdominant genes in the inheritance of quantitative traits, (b) the number of genes responsible for the genetic variation, and (c) the role of multiple peak epistasis in determining selection limits. In the first experiment, mass selection for pupa weight is used as a screening tool for eliminating the class of genes initially segregating that are not overdominant for pupa weight or pleiotropic in a negative way with reproductive fitness. Intense inbreeding, selection for combining ability, and statistical analyses of genetic variation will be used in plateaued populations to test for residual genetic variation and to determine whether there is any appreciable variation remaining that can be attributed to overdominant genes. The importance of multiple peak epistasis will be investigated in two ways. The first part of the work will make use of biometrical approaches to analyze the genetic variation in populations which are no longer responding to selection but where significant amounts of genetic variation remain. A triple test cross design will be used to determine (a) whether epistasis is a significant source of variation in plateaued populations and (b) whether populations derived from the same genetic base have been selected for different gene complexes. The second part of the experimental plan consists of a selection experiment designed to test specifically for multiple peak epistasis. The basic question is whether populations which contain the same sets of alleles for a number of genes but have different initial frequencies of these alleles will attain different plateaus if selection is the driving force in the evolutionary process.